The difficulty of measuring intelligence in non-human animals makes it a challenging topic for scientific study, however in general intelligence is defined as the process of acquiring, storing in memory, retrieving, combining, comparing, and using in new contexts information and conceptual skills.[1] The study of cephalopod intelligence has an important comparative aspect in the understanding of animal cognition because it relies on a nervous system fundamentally different from that of vertebrates.[2] In particular, the Coleoidea subclass (cuttlefish, squid, and octopi), are thought to be the most intelligent invertebrates and an important example of advanced cognitive evolution in animals, though nautilus intelligence is also a subject of growing interest among zoologists.[3]

The scope of cephalopod intelligence and learning capability is controversial within the biological community, complicated by the inherit complexity quantifying non-vertebrate intelligence. In spite of this, the existence of impressive spatial learning capacity, navigational abilities, and predatory techniques in cephalopods is widely acknowledged.[4]

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Cephalopods have large, well-developed brains[5], and their brain-to-body-mass ratio is the largest among the invertebrates, falling between that of endothermic and ectothermic vertebrates.[6]

The nervous system of cephalopods is the most complex of all invertebrates.[7] The giant nerve fibers of the cephalopod mantle have been widely used for many years as experimental material in neurophysiology; their large diameter (due to lack of myelination) makes them relatively easy to study compared with other animals.[8]

Unlike most other molluscs, all cephalopods are active predators (with the possible exceptions of the bigfin squid and vampire squid). Their need to locate and capture their prey has likely been the driving evolutionary force behind the development of their intelligence.[9]

Crabs, the staple food source of most octopus species, present significant challenges with their powerful pincers and their potential to exhaust the cephalopod's respiration system from a prolonged pursuit. In the face of these challenges, octopuses will instead seek out lobster traps and steal the bait inside. They are also known to climb aboard fishing boats and hide in the containers that hold dead or dying crabs.[10][11]

Captive cephalopods have also been known to climb out of their tanks, maneuver a distance of the lab floor, enter another aquarium to feed on the crabs, and return to their own aquariums.[12][13][14]

Many cephalopods are highly social creatures; when isolated from their own kind, some species have been observed shoaling with fish.[15]

Cephalopods are able to communicate visually using a diverse range of signals. To produce these signals, cephalopods can vary four types of communication elements: chromatic (skin coloration), skin texture (e.g. rough or smooth), posture, and locomotion. Changes in body appearance such as these are sometimes called polyphenism.[16] Some cephalopods are capable of rapid changes in skin colour and pattern through nervous control of chromatophores.[17] This ability almost certainly evolved primarily for camouflage, but squid use color, patterns, and flashing to communicate with each other in various courtship rituals.[16]Caribbean reef squid can even discriminate between recipients, sending one message using color patterns to a squid on their right, while they send another message to a squid on their left.[18][19]

The Humboldt squid shows extraordinary cooperation and communication in its hunting techniques. This is the first observation of such behavior in invertebrates.[20]

Octopuses have also been observed in what has been described as play: repeatedly releasing bottles or toys into a circular current in their aquariums and then catching them.[26]

Cephalopods can demonstrably benefit from environmental enrichment[27] indicating behavioral and neuronal plasticity not exhibited by many other invertebrates.

In a study on social learning, common octopuses (observers) were allowed to watch other octopuses (demonstrators) select one of two objects that differed only in color. Subsequently, the observers consistently selected the same object as did the demonstrators.[28]

At least four individuals of the veined octopus (Amphioctopus marginatus) have been observed retrieving discarded coconut shells, manipulating them, transporting them some distance, and then reassembling them to use as a shelter.[30] It is surmised that the octopuses originally used bivalves for the same purpose, before humans made coconut shells widely available on the sea floor.[31][32] Most hermit crabs use discarded shells of other species for habitation and other crabs choose sea anemones to cultivate on their carapaces as camouflage - as well, numerous insects use rocks, sand, leaves and other substrate materials as building materials; however, this behavior lacks the complexity of the octopus's fortress behavior, which involves picking up and carrying a tool to use later (however this argument remains contested by a number of other biologists who state that the shells actually provide continuous protection from abundant bottom-dwelling predators in their territory).[33]

Octopuses deliberately place stones, shells and even bits of broken bottle to form a wall that constricts the aperture to the den, another type of tool use.[34]

In laboratory studies, Octopus mercatoris, a small pygmy species of octopus, has been observed to block its lair using a plastic Lego brick.[35]

The highly sensitive suction cups and prehensile arms of octopuses, squid, and cuttlefish allow them to hold and manipulate objects. However, unlike vertebrates, the motor skills of octopuses do not seem to depend upon mapping their body within their brains, as the ability to organize complex movements is not thought to be linked to particular arms.[37]

Cephlopods can solve complex puzzles requiring pushing or pulling actions, and can also unscrew the lids of containers and open the latches on acrylic boxes in order to obtain the food inside. They can also remember solutions to puzzles and learn to solve the same puzzle presented in different configurations.[38]

Octopus opening a container with a screw cap

Captive octopi require stimulation or they will become lethargic; this typically takes the form of a variety of toys and puzzles.[39] At an aquarium in Coburg, Germany, an octopus named Otto was known to juggle his fellow tank-mates around, as well as throw rocks to smash the aquarium glass. On more than one occasion, Otto even caused short circuits by crawling out of his tank and shooting a jet of water at the overhead lamp.[40]

^Lee, Henry (1875). "V: The octopus out of water". Aquarium Notes – The Octopus; or, the "devil-fish" of fiction and of fact. London: Chapman and Hall. pp. 38–39. OCLC1544491. Retrieved 11 September 2015. The marauding rascal had occasionally issued from the water in his tank, and clambered up the rocks, and over the wall into the next one; there he had helped himself to a young lump-fish, and, having devoured it, returned demurely to his own quarters by the same route, with well-filled stomach and contented mind.